The present invention relates to a bookbinding device that sets sheets with images formed by an image forming apparatus or the like, applies an adhesive to the back of the sheet bundle, and binds a cover sheet to the sheet bundle. More specifically, the present invention relates to a bookbinding device that joins a sheet bundle with an adhesive applied to its back to the center of a front cover sheet, which is then back-folded for casing-in.
Common bookbinding devices of this kind include automatic bookbinding devices serving as terminal devices for image forming apparatuses such as printers and printing machines to stack sheets with images formed thereon so as to arrange the sheets in a bundle, apply an adhesive to end surfaces of the sheet bundle, and bind the sheet bundle to a cover sheet. Other devices of this kind include bookbinding devices that set printed sheets fed from a sheet feeding port and then bind the sheets to a cover sheet. In particular, for on-demand printing for electronic publishing or the like, known systems simultaneously perform a printing process and a bookbinding process by printing predetermined documents, while automatically binding the documents to a cover to form a book.
As an example of the above system, Japanese Patent Laid-Open Publication No. 2004-209867 discloses a device that automatically bookbinds sheets output by an image forming apparatus. According to this document, the device receives sheets output by an image forming apparatus through a sheet discharging port, guides the sheets to a sheet carry-in path, collects the sheets on a placement tray provided downstream of the path, rotates the sheet bundle in its horizontal posture collected on the tray through 90°, and guides the sheet bundle in its vertical posture to an adhesive pasting device for an applying process. The system device then folds the adhesive-pasted sheet bundle together with a cover sheet fed from an image forming apparatus or an inserter for binding.
A known method for bookbinding in these bookbinding systems is a casing-in operation involving joining a sheet bundle with an adhesive applied to its back to the center of a cover sheet carried in from a direction orthogonal to the sheet bundle, and folding the front cover together with the sheet bundle to form a book-like sheet bundle. In this case, the cover sheet with the sheet bundle joined to its center is pressed by a lateral pair of back folding blocks to form the back of the resulting book. Accordingly, the lateral pair of back folding blocks is shaped like a press die, and the back folding blocks, at respective standby positions located laterally away from each other relative to the central back folding position, approach each other to fold the back of the cover sheet at the central back folding position.
Japanese Patent Laid-Open Publication No. 2005-104063 discloses a technique for binding a cover sheet to a sheet bundle with an adhesive applied thereto and then adjusting a cooling time required to cool and solidify the adhesive in accordance with the thickness of the sheet bundle. According to this publication, the adhesive cooling time is set equal to the amount of time from the end of a bookbinding process until the beginning of the subsequent cutting process step.
As described above, when a hot-melt adhesive is applied to the back of the sheet bundle, which is then bound to the front sheet for bookbinding, the cooling time needs to be set in accordance with, for example, the thickness of the sheet bundle so that the adhesive is solidified before the subsequent step is reached. This is because a thicker sheet bundle requires a larger amount of adhesive applied and thus a longer cooling time. However, the adhesive cooling time is conventionally set equal to the time required to transfer the sheet bundle with the cover sheet bound thereto to the subsequent step. This disadvantageously increases the time required for a bookbinding process. That is, a thicker sheet bundle and a larger amount of adhesive applied increase the time required for a bookbinding process. Thus, for a continuous bookbinding process, the operation cannot be finished in time for the processing of the subsequent sheet bundle.
Thus, when a hot-melt adhesive is applied to the back of the sheet bundle, which is then bound to the front sheet, the applied adhesive needs to permeate through the sheets of the sheet bundle for reliable binding. This requires the adhesive to exhibit a high fluidity (low viscosity) at high temperatures. On the other hand, the high fluidity at high temperatures may cause the adhesive to leak to the back cover during pressing with the back folding blocks. In particular, pressing the back cover against a back rest plate causes an excessive amount of adhesive to leak, resulting in inappropriate bookbinding. In contrast, when the adhesive exhibits a low fluidity (high viscosity) at low temperatures, the back of the cover cannot be folded into sharp fold lines during pressing, or recesses and projections or crimps may be created on the back cover. Thus, desirably, when the sheet bundle and the cover sheet are bound together, the adhesive exhibits a high fluidity at relatively high temperatures during back folding pressing, and is rapidly solidified after the back folding. However, no improvements in the back folding mechanism based on the above knowledge have been made.